The overall goal of project 3 is to explore cardiovascular control mechanisms in human autonomic disorders. Our focus during this cycle will be to determine if the Postural Orthostatic Tachycardia Syndrome (POTS) is associated with fundamental alterations in vasoconstrictor function at rest and during exercise. Our major hypotheses are that a) adrenergic vasoconstriction is blunted in POTS, b) cardiac output, but not blood pressure, will rise normally during exercise in patients with POTS, and c) sympathetic restraint of blood flow to active muscles will be absent in POTS. Together these impairments will evoke excessive reflex increases in heart rate during orthostatic stress or dynamic exercise in patients with POTS.